On March 29, 2007, Columbia Banking
System, Inc. (Columbia), held a conference call during which it provided information regarding Columbias proposed acquisition of Mountain Bank Holding Company (and a separate proposed acquisition of Town Center Bancorp). A
transcript of the conference call is furnished as Exhibit 99.1 to this report and incorporated by reference.

Item 8.01.

Other Events

As reported in Columbias report on
Form 8-K filed March 29, 2007, Columbia and Mountain Bank Holding Company (MBHC) entered into a Plan and Agreement of Merger (the Merger Agreement) dated as of March 28, 2007, pursuant to which MBHC will merge
with and into Columbia, and MBHCs bank subsidiary, Mt. Rainier National Bank, will merge with and into Columbias subsidiary, Columbia State Bank. A copy of the Merger Agreement is filed as Exhibit 2.1 to this report and
incorporated by reference.

Item 9.01.

Financial Statements and Exhibits

(d) Exhibits.

2.1

Plan and Agreement of Merger dated as of March 28, 2007, among Columbia Banking System, Inc., Columbia State Bank, Mountain Bank Holding Company, and Mt. Rainier National Bank